Linewidth monitoring and control in lithography is a major concern for the semiconductor processing industry. Enormous equipment investment is needed in order to measure critical dimensions of the ultrafine features used in processing semiconductor devices. Lithographic printing steps in these processes are typically practiced using ultraviolet (UV) or deep ultraviolet (DUV) projection photolithographic patterning methods. Measuring the lines which these tools are capable of printing is increasingly difficult especially in view of the rapid pace at which the linewidths are shrinking.
In the manufacture of integrated circuits (ICs), patterns which are replicated onto semiconductor wafers are generally reduction printed from nX reticles (where n is typically 4 but can range from 1 to 20). The reticles are usually produced using electron beam lithography, in which either a round beam or a variable shaped rectangular beam of electrons is scanned over a resist-coated mask blank to expose the needed pattern. Linewidth control in reticle production has important consequences for control of the critical dimensions of the mask features, which are ultimately transferred to production wafers. Mask or reticle production is therefore another related industry for which linewidth monitoring is critical.
For a smaller segment of the semiconductor industry, the critical lithographic exposure is performed directly by a serial writing method such as electron beam lithography. These processes are generally referred to as direct-write processes. Such processes are often restricted to research use by cost and throughput limitations. However, techniques for improving the efficiency of direct-write processes are being sought, and linewidth measuring and control of the ultra fine features made using these processes will be essential for them to have significant commercial impact.
In each of the lithography industry sectors described above, an aerial image of actinic radiation is made incident on a resist material. The resist material is subsequently subjected to thermal and chemical treatment to produce the resultant resist profile. Because this is a multi-step process, many factors can and do cause variations in the dimensions of the features in the resist profile. For example, focus of the printing or writing tool, variations in resist baking temperatures and times, resist aging, time lapses between resist application and exposure and between exposure and post-exposure bake, and exposure dose variations, are all factors which can alter the resulting linewidth. It is therefore critical to monitor the final lithographic outcome as part of any quality control procedure.
In each of the application areas described above, considerable time, effort, and resources, are consumed in the inspection phase of the quality assurance process. Other high precision aspects relating to quality control measurements can be performed routinely using relatively low resolution inspection methods. For example, simple Vernier rulings are used to measure the level-to-level overlay accuracy in both UV lithography and electron beam lithography. The Vernier rulings have relatively large patterned features that can be viewed using an optical microscope, and it is relatively straightforward for an operator or automated video monitor to judge which of several rectangular features are most closely aligned. This simple procedure can determine the overlay error to within 0.05 .mu.m, about eight-fold smaller than the nominal diffraction limited resolution of the optical microscope.
Another example of monitoring for fine feature control is the use of Moire patterns to evaluate small magnification errors, distortions, and beam drift in electron beam writing tools. See, e.g., B. Hubner and H. W. P. Koops, J. Vac. Sci. Technol., B10 (1992), and references therein. In this method, tilted gratings or gratings of differing periods are overlays to produce "beat" patterns. These patterns can be used to measure small errors or changes, and are observable in an optical microscope.
The monitoring of critical dimensions has, however, been accomplished by more direct measurements using imaging methods with resolving power much finer than the features measured. This need to image and measure ever smaller linewidths has forced technology shifts in the measuring instruments, originally based on optical microscopy and migrating to scanning electron microscopy (SEM) and, more recently, atomic force microscopy (AFM).
A related time-domain problem was posed by the ultrafast laser pulse community in the late 1960s. The goal in that work was to measure the duration of a sub-100 picosecond pulse of light produced by a laser, a time scale too short to measure directly. One solution, described by H. P. Weber in J. Appl. Phys. 38, 2231 (1967) and 39, 6041 (1968), is known as the Second Harmonic Generation (SHG) Autocorrelation method. In this scheme the light beam is split and the two half-intensity beams are routed through different free space paths, one beam variably delayed with respect to the other. The two pulse beams are then reunited and passed through a non-linear optical medium capable of second harmonic generation. A detector tuned to the second harmonic radiation is then able to measure the overlap of the initial and delayed pulses. The waxing and waning of the second harmonic radiation intensity as the path length of the delayed beam is changed is a measure of the overlap of the two pulses. Using the path length changes to calibrate the time scale (.DELTA.t=.DELTA.d/c.sub.air) the correlation function is obtained, from which the pulse width is inferred.
It is evident from this background that a simple indirect method of inspection using an optical microscope for measuring and monitoring critical dimensions would enjoy widespread use in industries such as the semiconductor industry, and could provide routine non-invasive feedback for lithographers. The problem is to find that indirect optical method for determining sub-resolution linewidths.